Palladium(ii) ligated with a selenated (Se, CNHC, N-)-type pincer ligand: An efficient catalyst for Mizoroki-Heck and Suzuki-Miyaura coupling in water

Article abstract of DOI:10.1039/c9ob01674k

A new 1-[N-benzylacetamido]-3-[1-(2-phenylselenylethyl)]benzimidazolium chloride (L), the precursor of a novel (Se, C_NHC, N^-)-type pincer ligand (L) was synthesised in high yield through a sequence of consecutive reactions of 1H-benzimidazole with ethylene dichloride, sodium selenophenolate, and N-benzyl-2-chloroacetamide. The palladium-promoted reaction of L with PdCl₂ resulted in a moisture- and air-insensitive complex [Pd(L-H₂Cl)Cl] (1), which demonstrated outstanding catalytic potential for Mizoroki-Heck coupling of aromatic bromides and chlorides (with yields up to 94% and 70%, respectively) at very low catalyst loading (0.2 mol%) and under mild reaction conditions in water. The complex (1) was also investigated for Suzuki-Miyaura coupling and found to be selectively efficient (yields up to 94%) for Suzuki-Miyaura coupling of aromatic bromides at 0.01 mol% of 1 in water. All coupling reactions were carried out in the green and economical solvent, water, which is highly desirable for bulk synthesis of complex molecules in industry. During the catalytic process, complex 1 converted into PdSe nanoparticles (NPs, size range 5-6 nm) in situ. The morphology and composition of these NPs were analysed through high-resolution transmission electron microscopy and transmission electron microscopy-energy dispersive X-ray spectroscopy, respectively. The core-level, X-ray photoelectron spectroscopy analysis confirmed the presence of stable Pd⁰ and Pd²⁺ oxidation states in these PdSe NPs. Based on further experimental investigations, these nanoparticles were found to work as a stock of true catalytic species. The hot filtration test, as well as the two-phase test, confirmed the largely homogeneous nature of the catalytic process, which probably proceeds by leaching of solution-phase Pd species from these NPs.

Full text of DOI:10.1039/c9ob01674k

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DOI: 10.1039/C9OB01674K
ARTICLE
Palladium(II) ligated with selenated NHC based (Se, CNHC, N−) type
pincer: An efficient catalyst for Mizoroki-Heck and Suzuki Miyaura
coupling in water
Received 00th January 20xx,
Accepted 00th January 20xx
Kamal Nayan Sharma,*a Naveen Satrawala,a Avinash Kumar Srivastava,a Munsaf Ali,a and Raj
DOI: 10.1039/x0xx00000x
Kumar Joshi*a
A
new 1-[N-benzylacetamido]-3-[1-(2-phenylselenylethyl)] benzimidazolium chloride (L), the precursor of a novel
(Se,CNHC,N−) type first pincer ligand (L) has been synthesized in high yield through a sequence of consecutive reactions of
1H-benzimidazole with ethylene dichloride, sodium selenophenolate, and N-benzyl-2-chloroacetamide. The palladium
promoted reaction of L with PdCl2 has resulted in a moisture and air insensitive complex [Pd(L−H2Cl)Cl] (1) which has
demonstrated outstanding catalytic potential for Mizoroki-Heck of aromatic bromides/chlorides (Yields up to 94%/70%) at
very low catalyst loading (0.2 mol%) under mild reaction conditions in water. The complex 1 has also been explored for the
Suzuki-Miyaura coupling and found to be selectively efficient (Yields up to 94%) for Suzuki-Miyaura coupling of aromatic
bromides at 0.01 mol% of 1 in water. All coupling reactions were carried out in green and economical solvent water, which
is highly desired for bulk synthesis of complexed molecules in industries. During the catalytic process, the complex 1 in situ
converted into Pd-Se nanoparticles (NPs, size range 5-6 nm). The morphology and composition of these NPs were analysed
through HR-TEM and TEM-EDX, respectively. The core-level X-ray photoelectron spectroscopic (XPS) analysis has
confirmed the presence of stable Pd0 and Pd+2 oxidation states in these Pd-Se NPs. Based on further experimental
investigations, these nanoparticles were found to work as a stock of true catalytic species. The hot-filtration, as well as
two-phase test, have confirmed the largely homogeneous nature of the catalytic process which most probably proceeds by
leaching of solution-phase Pd species from these NPs.
insensitive catalysts those can efficiently surpass the
challenges of sophisticated catalysts and can work in non-
flammable and less noxious green solvents. According to the
Introduction
The Mizoroki-Heck reaction1, 2 has grown as an indispensable
and prominent standard protocol for organic synthesis in the
industry as well as academia.3, 4 Various synthetic applications
of the Mizoroki-Heck coupling for the synthesis of olefinic
cross-coupled products which are the main building blocks of
wide range of complex molecules including natural products
traditional proponents of principles of green chemistry,12
water is the greenest and best alternative solvent for organic
solvents.13-15 Owing to the unique two-electron σ-donor and
negligible ᴫ-acceptor properties, NHCs usually form the metal
complexes with an enhanced thermal and air/moisture
stability and also allow access to the metal complexes,
outstanding in the catalysis.16-18 From the last several decades,
NHCs are enjoying the status of most indispensable tool in
catalyst design in organometallic catalysis. The investigations
of NHCs are mainly on track after the first metal-NHC
complexes reported by Wanzlick,19 Öfele,20 Lappert,21 and the
isolation of the first free NHC by Arduengo22 in 1991. The
pincer frame-work designed using NHC ligands are known as a
useful motif to provide electronic, topological versatility and
protective sheltered environment to the metal centre which in
turn affords their metal complexes, high stability, and catalytic
activity.23 An assorted collections of N, P donor functionalized
NHC based pincer type such as CNC,24 CCC,25 PCP,26 and CNC27
have been shown the terrific catalytic activity for organic
reactions. Comparable to the NHC, the organochalcogen
ligated metal complexes have also been recognized as
potential candidates for the catalysis of Suzuki-Miyaura
and fine chemicals are worth mentioning.5, Several bulky
6
electron-rich phosphine based palladium catalysts,7-9 and
palladacycles8, 10, 11 catalysts have been reported for the Heck
coupling but the certain drawbacks are associated with them
such as the use of toxic solvents, strongly desired air/moisture-
free conditions and limited reactivity for readily available
inexpensive aryl chloride as substrates. Hence, due to the
economic concerns and environmental sustainability, there is a
high demand to develop the economic, air/moisture
a. Department of Chemistry, Malaviya National Institute of Technology Jaipur, J.L.N.
Marg, Jaipur 302017, Rajasthan, India .
* Corresponding authors: rkjoshi.chy@mnit.ac.in, kamalnayaniitd@gmail.com
†
Electronic Supplementary Information (ESI) available: NMR spectra, mass
spectra, crystal and refinement data, bond lengths and bond angles, NMR data of
all the Mizoroki-Heck coupling products and CIF of 1 (CCDC 1876803) are provided
as ESI.
See DOI: 10.1039/x0xx00000x
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and 1,
coupling,28 Mizoroki-Heck coupling,29 and Sonogashira
coupling30 reactions. The previous reports on metal complexes
of organochalcogen pincer ligands31 have also shown their
out-performance to the phosphine based analogue.9 The
functionalizations of NHC with Se donor side arms have been
proven to be advantageous in the further improvement of
catalytic properties of their metal complexes.30, 32, 33 Therefore,
it seems worthwhile to explore the coordination behaviour
and catalytic potential of Se containing NHC pincer ligand. To
the best of our knowledge, the pincer framework constructed
with the combination of Se, NHC and N− donors is not reported
yet. Hence, it is the first report of a novel, unique selenated-
NHC pincer complex. Here, a new structurally defined, robust,
easy-to-handle Pd(II) complex of (Se,CNHC,N−) type pincer was
synthesized and used to activate the bromo- and chloroarenes
for the Mizoroki-Heck and Suzuki-Miyaura coupling reactions
in water.
450.1073 and 553.9953 in mass spectra oVfiew Article Online
L
respectively, correspond to [M−Cl]+.32 Finally, the molecular
DOI: 10.1039/C9OB01674K
structure of 1 was confirmed by single-crystal X-ray analysis
(Table S1 of SI). The thermal ellipsoid diagram of 1 is depicted
in Figure 1 (details in Table S2 of ESI†). Here, the ligand is
coordinated to metal centre in a unique tridentate pincer
(Se,CNHC,N−) mode and formed two six-member chelate rings.
The geometry is nearly square planar and the Pd-CNHC, and Pd-
Se bond lengths are 1.935(6) and 2.4046(8) Å, respectively,
which are in close agreement with the reported 1.957(10) and
2.3909(15) Å values, respectively.32
Results and discussion
The strategy used for the synthesis of the benzimidazolium salt
(L) and Pd(II) complex [Pd(L−H2Cl)Cl] (1), is depicted as Scheme
1.
Fig. 1 Thermal ellipsoid diagram of 1 at the 30% probability level. H
atoms and CHCl3 molecule are omitted for clarity. bond lengths (Å):
Pd(1)–C(1) 1.935(6), Pd(1)–Se(1) 2.4046(8); Bond angles (°): C(1)–
Pd(1)–Se(1) 92.73(17), C(1)–Pd(1)–N(1) 85.5(2).
Se Na+
N
N
Cl
NaBH4 (2 equiv)
Ph2Se2
2
reflux, ethanol
N2 atm, 15 min
reflux
N
N
Se
A
Catalytic investigations of 1 for Mizoroki-Heck coupling reaction
O
toluene
reflux, N2 atm
8 h
Cl
5
The Mizoroki-Heck coupling reaction, which is a very well
established palladium-catalysed cross-coupling reaction, has
been continuously enjoying the bottomless interest from both
the industry as well as academia since the last several
decades.1-4, 34 In view of air/moisture sensitivity of phosphine
based catalysts, palladium complexes of phosphine-free
ligands for the Mizoroki-Heck coupling reaction are of much
interest. The complex 1 has been explored for the catalysis of
the Mizoroki-Heck reaction (Scheme 2) of several
combinations of aryl halides and alkenes.
N
H
4
N
N
6
3
2
13
12
(CH3CN)2PdCl2
DMF, N2 atm, 90 °C
7
N
9
10
Se
O
O
16
N
Pd Se
Cl
+
N
11
N
H
15
17
18
8
14
1
20
Cl
19
1
L
Scheme 1 Synthesis of benzimidazolium chloride (L) and [Pd(L−H2Cl)Cl]
(1).
1-(2-(Phenylselanyl)ethyl)-1H-benzimidazole (A) was prepared
R
R
1
by an earlier reported procedure.30 The H NMR, 13C{1H} NMR
Aryl / heteroaryl
R = H, Me
or
and mass spectra have authenticated the formation of L and 1,
(ESI†, Figures S1-S6,). The selenoether-NHC precursor (L) was
synthesized through the atom economy reaction of A with N-
benzyl-2-chloroacetamide in toluene. Finally, the direct Pd
promoted the reaction of NHC precursor (L) and PdCl2 in DMF
resulted the complex 1. The 1H, 13C{1H} NMR and mass spectra
of L (ESI†, Figure S1-S3) and 1 (ESI†, Figure S4-S6) confirmed
the formation of both the benzimidazolium chloride and Pd(II)
1
Complex
K3PO4, TBAB
Water, 110 o
+
X
Aryl / heteroaryl
X = Br, Cl
C
ROOC
ROOC
Aryl / heteroaryl
R = Et, n-Bu
Scheme 2 Mizoroki-Heck coupling reaction catalysed by 1.
The present Pd(II)-NHC (1) was found highly active for the
Mizoroki-Heck coupling of aryl/heteroaryl bromides as the
yield of coupling products up to 94% were obtained at very
low catalyst loading (0.2 mol%) in neat water as a solvent. The
catalyst was also found significantly active for the coupling of
aryl chlorides and quite a good yield up to 70% was obtained.
The preliminary standardisations were performed using the
Mizoroki-Heck coupling of 4-bromoacetophenone and styrene
as a representative example (Table S3-S5 in ESI†).
1
complex. In the H NMR of L, a singlet obtained at 10.51 ppm,
indicates the highly deshielded proton (H1) attached with the
pre-carbene carbon (C1) and it is in the range of previously
reported imidazolium salts (δ 9.5-12.0 ppm), and it also
suggests the high acidity of the proton.32 A triplet for NH
proton of amido group was recorded at 9.85 ppm, further
confirmed the formation of desired compound L. The absence
of both these signals in the 1H NMR of 1, hint the
deprotonation of C1 proton and NH (amide) due to the
coordination with Pd(II). A most intense peak at (m/z)
2 | J. Name., 2012, 00, 1-3
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To examine the scope of the present coupling reaction,
different aryl and heteroaryl bromides and chlorides with 4-
methyl styrene, styrene, n-butyl acrylate and ethyl acrylate
were explored (Table 1-2). It appears that the catalyst was
potentially active for both the activated as well as deactivated
substrates. However, the lower yields of desired stilbene
product were delivered in case of aryl chlorides than their
analogues aryl bromides. The trans isomer of 1,2-disubstituted
alkenes was appeared to be the sole product in all the
reactions. During the present Heck coupling reactions, an
appearance of black material was observed just after initiating
the catalytic reaction. In order to find out the role of this black
material, a typical reaction of 4-bromoacetophenone with
styrene was examined methodically (for details, see
experimental section). The black particles formed during the
reaction were isolated and subjected to characterisation by
HR-TEM (Figure 2) and TEM-EDX (Figure S7 of ESI†). The HR-
TEM shows the uniformly dispersed spherical shaped PdSe
NPs. The size of these NPs was observed in the range of 5-6 nm
(75%). The ratio of Pd : Se calculated based on atom%
estimated through quantitative TEM-EDX, which is nearly 7 : 2.
The selected area (electron) diffraction pattern (SAED) shows
the poorly crystalline nature of these NPs (Fig. 2d).
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DOI: 10.1039/C9OB01674K
13
5-Bromopyrimidine
82
14a 2-Bromothiophene
14b 2-Chlorothiophene
78
48
15a 3-Bromothiophene
15b 3-Chlorothiophene
78
48
Reaction conditions: Aryl halide (1.0 mmol), 4-methylstyrene
(0.177 g, 1.5 mmol), K3PO4 (0.319 g, 1.5 mmol), TBAB (0.5 mmol),
catalyst (0.002 mmol, 0.2 mol%), water (1.5 mL), Temp 110 °C, time
12 h and yields (isolated after column purification).
Furthermore, to ascertain the oxidation state of Pd, a core-
level X-ray photoelectron spectroscopy (XPS) measurement
was conducted (Figure 2f). The appearance of two main
doublets in the XPS spectrum indicates the presence of two
oxidation states of Pd. The lower energy doublet peaks at
335.5 (3d5/2) and 340.8 (3d3/2) could be ascribed to the Pd0
while the higher binding energy doublet peaks at 337.8 (3d5/2
)
and 342.8 (3d3/2) was ascribed for +2 oxidation state of Pd in
these NPs.35
Table 2 Heck reactions of aryl halides with styrene, n-butyl acrylate
and ethyl acrylate catalysed by 1.
Table 1 Mizoroki-Heck reactions of aryl-bromides/chlorides with 4-
methyl styrene catalysed by 1
Yield
(%)
Entry Aryl Halide
Heck Coupling Product
Yield
(%)
1a
1b
4-Bromobenzene
4-Chlorobenzene
94
70
Entry Aryl/Heteroaryl Halide
Heck Coupling Product
1a
1b
4-Bromoacetophenone
4-Chloroacetophenone
90
62
2a
2b
4-Bromoacetophenone
4-Chloroacetophenone
92
62
2a
2b
4-Bromobenzaldehyde
4-Chlorobenzaldehyde
88
61
3a
3b
1-Bromo-4-nitrobenzene
1-Chloro-4-nitrobenzene
90
60
3a
3b
1-Bromo-4-nitrobenzene
1-Chloro-4-nitrobenzene
85
62
4a
4b
4-Bromobenzonitrile
4-Chlorobenzonitrile
85
62
4a
4b
4-Bromobenzonitrile
4-Chlorobenzonitrile
88
65
5a
5b
4-Bromotoluene
4-Chlorotoluene
94
68
5a
5b
4-Bromobenzoic acid
4-Chlorobenzoic acid
75
60
6a
6b
Bromobenzene
Chlorobenzene
94
68
6a
6b
4-Bromobenzotrifluoride
4-Chlorobenzotrifluoride
88
64
7a
7b
1-Bromo-4-nitrobenzene
1-Chloro-4-nitrobenzene
92
64
7a
7b
Bromobenzene
Chlorobenzene
92
68
8a
8b
4-Bromobenzene
4-Chlorobenzene
94
62
8a
8b
4-Bromotoluene
4-Chlorotoluene
92
55
Aryl halide (1.0 mmol), alkene (1.5 mmol), K3PO4 (0.319 g, 1.5
mmol), TBAB (0.5 mmol), catalyst (0.002 mmol), water (1.5 mL),
Temp 110 °C, time 12 h and yields (isolated after purification).
9a
9b
4-Bromoanisole
4-Chloroanisole
82
42
To investigate the re-usability of NPs, the isolated NPs
were used for a fresh Heck reaction of 4-bromoacetophenone
and styrene. The NPs were found to be active for the reaction,
but the efficiency was drastically reduced after one cycle. To
find out the nature of catalytic process (homogeneous or
heterogeneous), a hot filtration test was conducted (for
details, see experimental section). Here, the maximum
conversion was reached up to 75% even after 12h of the
reaction. It shows that the reaction is somewhat quenched in
10a 2-Bromopyridine
10b 2-Chloropyridine
85
55
11a 3-Bromopyridine
11b 3-Chloropyridine
72
45
12
3-Bromoquinoline
86
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the filtrate as the maximum conversion dropped from 92% to
75%. The 75% conversion was performed by the palladium
species which leached into solution from in situ generated
NPs. All these observations indicate a largely homogeneous
nature of catalytic process, which is most probably due to
leaching of Pd(0)/(II) from these Pd-Se NPs. Furthermore when
isolated NPs were reused to catalyse the Heck coupling, their
efficiency reduced drastically after one use. These results are
also in agreement with the conclusion drawn from the hot
filtration test because most of the catalytically active Pd has
been leached out in solution in first run.
phase test experiment, ~90% yield of cross-coupled product of
4-bromoacetophenone and styrene from solution phase was
View Article Online
DOI: 10.1039/C9OB01674K
obtained. The 75% yield of the cross-coupled product of
immobilised 4-bromobenzoic acid (as amide) was isolated after
the hydrolysis of the solid phase of the reaction mixture which
suggests that the leaching of the homogeneous molecular or
colloidal Pd species from Pd-Se NPs responsible for carrying
out the catalytic process mostly in homogeneous fashion.
O
O
Cl
pyridine
N
NH2
Br
Si
Si
+
Si
H
Br
12 h, 40 °C
sustrate immobilized onto silica
O
O
Br
+
+
N
H3C
H
Br
(i) 1 (0.2 mol%), water
K3PO4, TBAB, 110 °C
(ii) KOH, EtOH-water
H3COC
HOOC
HOOC
Br
+
+
25%
90%
75%
Scheme 3 Mizoroki-Heck coupling reaction related to the two-phase
test.
Suzuki-Miyaura coupling reaction
The catalytic potential of the present catalyst was also
investigated for Suzuki-Miyaura cross-coupling of aryl halides.
Several aryl bromides and chlorides were tested for Suzuki
Miyaura coupling with phenylboronic acid, p-tolylboronic acid
and 4-fluorophenylboronic acid in water under aerobic
reaction conditions. Various trials were attempted for the
Suzuki-Miyaura cross-coupling of 4-chloroacetophenone and
4-chlorobenzaldehyde and phenylboronic acid, but
unfortunately, we did not observe any trace of the coupling
product. While shifting from aryl chloride to aryl bromide, at
similar reaction conditions, the good yields of cross-coupling
products were obtained (Table 3). This indicates the catalyst
work effectively for the Suzuki-Miyaura coupling of aryl
bromides and aryl boronic acid.
Fig. 2 TEM micrographs at (a) 50 nm, (b) 10 nm, and (c) 5 nm scale bar,
(d) selected area (electron) diffraction (SAED) pattern, (d) size
distribution of the NPs and (f) palladium XPS analysis of the Pd-Se NPs.
Table 3 Suzuki-Miyaura coupling reactions of aryl/heteroaryl-bromides
with phenylboronic acid catalysed by 1.
Entry Aryl/Heteroaryl Halide
Suzuki Coupling Product
Yield (%)
In order to further confirm the heterogeneous vs
homogeneous nature of catalysis, two-phase test36 was carried
out (Scheme 3). During the two-phase test reaction, for a
heterogeneous catalyst, the immobilised substrate should not
be converted to the cross-coupled product, but when Pd is
released into solution phase (i.e., the nature of catalysis is
homogeneous), the substrate immobilised onto the solid
surface can also be converted to the product.
1
1’
4-Bromoacetophenone
4-Chloroacetophenone
93
nd
COCH3
2
2’
4-Bromobenzaldehyde
4-Chlorobenzaldehyde
94
nd
CHO
CN
3
4
5
4-Bromobenzonitrile
4-Bromoanisole
93
86
90
OCH3
NO2
A solution-phase aryl halide (4-bromoacetophenone) was used
to confirmed the presence of a catalytic process to occur
which is expected to be converted into cross-coupled product
in both conditions, irrespective to the nature of the catalytic
process homogeneous or heterogeneous. In the present two-
1-bromo-4-nitrobenzene
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N
N
Experimental Section
Physical measurements
View Article Online
DOI: 10.1039/C9OB01674K
6
7
5-Bromoprimidine
3-Bromoquinoline
88
87
The 1H and 13C NMR spectra were recorded on a Bruker
Spectrospin DPX-400 NMR spectrometer and JEOL ECS-400
spectrometer. The FT-IR spectra were recorded on KBr pellets
in the range of 4000-400 cm−1 on a Perkin Elmer 10.4.00
spectrometer. Xevo G2-S Q-Tof (Waters, USA) instrument was
used for High-Resolution Electron Impact Mass Spectra (HR-
EIMS). Single crystal X-ray analysis was carried out on a CCD
Agilent Technology Supernova diffractometer equipped with a
low-temperature attachment. Data collection at 150(2) K using
graphite-monochromatic Mo Kα radiation (λα = 0.71073 Å) was
performed. The strategy for the data collection was evaluated
by using the CrysAlisPro CCD software. Data were collected by
the standard phi-omega scan techniques and were scaled and
reduced using CrysAlisPro RED software. The structure of 1
was solved by direct methods using SHELXS-97 and refined by
full-matrix least-squares with SHELXL-97, refining on F2.42 One
disordered solvent molecule was removed from all scattering
by using the PLATON/SQUEEZE program.43 Direct methods
were used to obtain the positions of all the atoms. All non-
hydrogen atoms were refined anisotropically. The high-
resolution TEM (HR-TEM) characterisation of NPs was
performed with a Philips Tecnai F20 operated at 200 kV.
Samples for HR-TEM were prepared by drying a drop of
dispersed NPs in ethanol on 400 mesh size carbon-coated
copper grid. The elemental composition of NPs was studied
with EDX system associated with HR-TEM, Model QuanTax
200. X-ray photoelectron spectroscopy (XPS) spectrum was
recorded on ESCA+ omicron nanotechnology oxford
instruments. The melting points were determined by open
capillary and reported as obtained. All the reaction products
were vacuum dried for several hours before being analysed
and weighted as well.
N
H3C
H3C
F
CH3
NO2
8
4-Bromotoluene
91[a]
92[a]
82[b]
9
1-bromo-4-nitrobenzene
1-bromo-4-nitrobenzene
NO2
10
Reaction conditions: Aryl halide (1.0 mmol), boronic acid (1.5 mmol),
K2CO3 (0.276 g, 1.5 mmol, catalyst (0.01 mol%), water (1.5 mL), Temp
55 °C, time 2 h, yields are isolated after column purification, for Entries
1’ and 2’ Temp is 110 °C and catalyst loading was also increased upto
0.2 mol%; [a] p-tolylboronic acid and [b] 4-fluorophenylboronic acid.
In recent past, we have reported a Pd(II) complex of sulfur analogue
of the present selenoether-NHC ligand and explored it for the
Suzuki-Miyaura cross-coupling of aryl/heteroaryl bromides.37 The
thioether-NHC ligand was coordinated with Pd(II) in a bidentate
half-pincer (S,CNHC) mode while the amide functionality present at
another nitrogen of NHC left intact and not coordinated to Pd(II). In
view of catalytic activity, it was found to be selectively efficient for
Suzuki-Miyaura coupling of aryl bromide, but it was found inactive
for the coupling of latent aryl chloride substrates. Furthermore, the
thioether-NHC Pd complex failed to mimic Heck coupling of aryl
bromides/chloride. The selective reactivity of sulfur analogue
complex may be due to its half pincer mode coordination with
Pd(II); moreover, the high electronegativity of S made it weaker
donor as compare to Se. While the present complex reflects a
unique combination of donor sites within the pincer (Se,CNHC,N−)
environment, it was found very active for Heck coupling of aryl
bromides as well as chlorides. However, it also failed to show
Suzuki-Miyaura coupling of aryl chlorides but found significantly
active for the coupling of aryl and heteroaryl bromides. As far
literature concern, it is quite evident that the full pincer Pd(II)
Chemicals and reagents
Palladium(II)
chloride,
diphenyl
diselenide,
sodium
complexes have high catalytic efficiency as compared to the half-
borohydride and 3-aminopropyl trimethoxysilane-modified
pincer complexes.29,
The high catalytic efficiency of selenium
based ligands to their sulfur analogue was also experimentally and
theoretically justified by Singh and co-workers.31,
reports are in close agreement with increased catalytic activity of
the present Se based Pd(II) catalyst over the previously reported
sulfur analogue Pd catalyst.
38
silica procured from Sigma-Aldrich (USA) were used as
received.
N-benzyl-2-chloroacetamide44
and
1-(2-
39-41
phenylselenyl-ethyl)-1H-benzimidazole30 were prepared by
previously reported methods. Standard methods were used
for drying and distillation of the solvents.45 Silica gel (60-120
and 100-200 mesh) and TLC sheets (silica gel 60F254) used in
column chromatography, were procured from Fisher Scientific
and Merck, respectively. Commercially available common
chemicals and reagents were procured within the country.
All these
Additionally, a comparison of the catalytic efficiency of present
catalyst 1 with the reported P/N donor functionalized NHC–Pd
catalysts for Heck coupling is provided in ESI† (Table S6). It
appears that the present catalyst 1 is efficient because the
yield up to 94% was obtained in 12 h with 0.2 mol% loading of
1 under aqueous/aerobic reaction conditions. The unique
combination of donor sites within the pincer environment in
which the Pd metal is located are probably responsible for the
high catalytic activity of 1 and made the present catalyst a
Synthesis
of
1-[benzylaminocarbonylmethyl]-3-(2-
phenylselenylethyl)benzimidazolium chloride (L)
A mixture of 1-(2-phenylselenylethyl)-1H-benzimidazole (A,
0.301 g, 1.0 mmol) and N-benzyl-2-chloroacetamide (0.183 g,
1.0 mmol) in toluene (10 mL) was stirred under reflux in an
inert atmosphere to give L as a white solid, which precipitated
in the reaction medium after 12 h. The reaction mixture was
viable alternative over
a classical air/moisture-sensitive
phosphine-based Pd catalysts.
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filtered, and the obtained residue was washed with toluene mL) and dried over anhydrous Na2SO4. The solvent of the
View Article Online
DOI: 10.1039/C9OB01674K
and dried in vacuo to yield L as a white solid.
extract was evaporated off on a rotary evaporator, and the
crude product was further purified by column chromatography
on silica gel (100-200 mesh) using ethyl acetate-hexane as
eluent. All coupling products were authenticated by 1H and
13C{1H} NMR spectra matched with literature reported data
1
Yield: 0.497 g (94 %). M.p. 126 °C. H NMR (400 MHz, CDCl3) δ
3
(ppm) 10.51 (s, 1H, H1), 9.85 (t, JH−H = 5.8 Hz, 1H, NH amide),
7.84 – 7.76 (m, 1H, H5), 7.50 – 7.39 (m, 2H, H3 and H6), 7.36 –
7.31 (m, 2H, H19), 7.31 – 7.26 (m, 1H, H4), 7.23 (d, JH−H = 7.2
3
except
the
three
compounds
4'-Methyl-trans-4-
3
Hz, 2H, H12), 7.18 – 7.06 (m, 5H, H11, H20, H18), 7.03 (t, JH−H
=
stilbenecarboxylic acid (Table 4, Entry 5), (E)-3-(4-
Methylstyryl)quinoline (Table 4, Entry 12) and (E)-5-(4-
Methylstyryl)pyrimidine (Table 4, Entry 13) which are not
reported yet. Their H NMR and 13C NMR spectra were found
to be consistent with the concerned structure given for them
in Table 4 (Entries 5, 12 and 13).
7.3 Hz, 1H, H13), 5.51 (s, 2H, H14), 4.60 (t, 3JH−H = 6.8 Hz, 2H, H8),
3
3
4.30 (d, JH−H = 5.6 Hz, 2H, H16), 3.35 (t, JH−H = 6.8 Hz, 2H, H9).
13C{1H} NMR (101 MHz, CDCl3) δ (ppm) 164.42 (C15), 142.88
(C1), 138.04 (C17), 133.47 (C18), 131.76 (C10), 130.63 (C7, C2),
129.41 (C19), 128.36 (C11), 128.05 (C6), 127.79 (C12), 127.32 (C3),
127.01 (C13, C20), 114.56 (C5), 112.04 (C4), 49.78 (C14), 47.69
(C8), 43.50 (C16), 25.70 (C9). Mass (CH3CN) [M − Cl]+ (m/z)
Found: 450.1073; Calc. value for [C24H24N3OSe]+: 450.1079. FT-
IR (KBr, νmax/cm−1): 3207 (m, νN–H amide), 3056 (m, νC–H aromatic),
2913 (m, νC–H aliphatic), 1681 (s, νC=O), 1556 (s, νC=C aromatic), 1253
(m, νC‒N aliphatic), 759 (s, νC–H aromatic, bending).
1
Isolation of in situ formed Pd-Se NPs from Mizoroki-Heck
coupling reaction
A
reaction tube containing the mixture of 4-
bromoacetophenone (0.199 g, 1.0 mmol), styrene (0.156 g, 1.5
mmol), K3PO4 (0.424 g, 2.0 mmol), tetra-n-butylammonium
bromide (0.161 g, 0.5 mmol) and catalyst (0.235, 0.4 mmol) in
water (2.0 mL) was stirred at 110 °C for 12 h. Thereafter, the
solvent water was decanted off and black material was washed
with acetone/water, dried in vacuum and subjected to
analysis.
Synthesis of [Pd(L–HCl)Cl] (1)
A mixture of L (0.248 g, 0.5 mmol) and PdCl2 (0.089 g, 0.5
mmol) in DMF (5 mL) was stirred under an inert atmosphere at
90 °C for 12 h. Thereafter, the solvent was removed by using a
rotary evaporator and the residue was dissolved in a minimum
amount of dichloromethane. After the addition of diethyl
ether, a crude yellow solid precipitated which was isolated by
filtration and purified through column chromatography on
silica gel using DCM/methanol as eluent. The obtained light
orange solid was dried under vacuum and subjected for
analysis. The suitable quality single crystals were grown by the
slow evaporation of a saturated solution of 1 in CH2Cl2-CH3OH
Reuse of isolated Pd-Se nanoparticles for catalysis of
Mizoroki-Heck coupling reaction
The Mizoroki-Heck coupling reaction of 4-bromoacetophenone
with styrene in the presence of 10 mg of isolated Pd-Se
nanoparticles under similar reaction conditions optimised for
catalyst 1 was carried out. The isolated yield of the cross-
coupled product was found to be ~35%.
1
(4:1) at 0 °C. Yield: 0.259 g, 88 %. M.p. 185 °C. H NMR (400
MHz, CDCl3) δ (ppm) 7.59 – 7.54 (m, 3H, H3, H5, H6), 7.44 – 7.32 Hot-filtration test
(m, 5H, H4, H19, H12), 7.27 – 7.18 (m, 6H, H11, H20, H18, H13), 5.37
The hot filtration test was employed in the Heck reaction of 4-
– 5.32 (m, 1H, H8), 5.10 – 4.97 (m, 2H, H14), 4.79 – 4.75 (m, 2H,
H16), 4.369 – 4.38 (m, 1H, H8), 4.13 – 4.00 (m, 1H, H9), 2.89 –
2.61 (m, 2H, H9). 13C{1H} NMR (101 MHz, CDCl3): δ (ppm)
165.98 (C15), 163.18 (C1), 143.39 (C17), 134.91 (C18), 132.96
(C10), 132.55 (C7, C2), 130.27 (C19), 129.97 (C11), 129.02 (C6),
127.85 (C12), 125.86 (C3), 124.88 (C13), 124.36 (C20), 111.67 (C5),
110.25 (C4), 53.03 (C14), 49.72 (C8), 44.35 (C16), 31.46 (C9). Mass
(CH3CN) [M − Cl]+ (m/z) Found: 553.9953; Calc. value for
[C24H22N3OPdSe]+: 553.9957. FT-IR (KBr, νmax/cm−1): 3055 (m,
νC–H aromatic), 2912 (m, νC–H aliphatic), 1587 (s, νC=O), 1576 (s, νC=C
aromatic), 1283 (m, νC‒N aliphatic), 749 (s, νC–H aromatic, bending).
bromoacetophenone and styrene in the presence of 1 under
the optimal reaction condition to ascertain the nature of
catalysis. For this purpose,
a mixture containing 4-
bromoacetophenone (0.199 g, 1.0 mmol), styrene (0.156 g, 1.5
mmol), K3PO4 (0.424 g, 2.0 mmol), tetra-n-butylammonium
bromide (0.161 g, 0.5 mmol), catalyst (0.2 mol%) and water
(2mL) in a reaction tube was stirred at 110 °C and after 30 min
it was filtered hot through a G-4 crucible having a thin celite
pad. The filtrate was further stirred at 110 °C for next 12 h. The
conversion was monitored in the filtrate with time, and
isolated product was analysed by 1H NMR.
Procedure for Mizoroki-Heck coupling
Two-phase test
A reaction tube charged with aryl/heteroaryl halide (1.0
mmol), alkene (1.5 mmol), K3PO4 (0.424 g, 2.0 mmol), tetra-n-
The immobilized silica substrate required for this test was
prepared by a literature reported procedure.46 In a reaction
butylammonium bromide (0.161 g, 0.5 mmol), water (2.0 mL), tube, 4-bromobenzoic acid-immobilized on silica gel as amide
(0.20 g), 4-bromoacetophenone (0.20 g, 1 mmol), styrene
(0.312 g, 3 mmol), K3PO4 (0.848 g, 4 mmol) and catalyst (2.352
mg, 0.2 mol%) were taken in 2 mL water and the mixture was
stirred at 110 °C for 12 h. Thereafter, it was cooled to room
temperature and filtered through a G-4 crucible. The residue
was washed with water (20 mL) followed by ethyl acetate (15
mL). The filtrate and washings were collected together and
and catalyst (1.176 mg, 0.002 mmol, 0.2 mol%) was placed on
an oil bath maintaining temperature at 110 °C. The reaction
mixture was stirred and monitored the reaction by TLC until
the maximum conversion of the substrate reached out. After
the optimal conversion indicated on TLC, the reaction was
stopped, and the reaction mixture was extracted with ethyl
acetate (2 × 15 mL). The extract was washed with water (2 × 15
6 | J. Name., 2012, 00, 1-3
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mixed with 50 mL of water. The resulting mixture was
extracted with ethyl acetate (50 mL). The solvent of the extract
was evaporated off on a rotary evaporator, and the residue
RKJ, and NS thank DST New Delhi for to provideVtiehweArrteicsleeOanrlcinhe
funds. KNS thanks to SERB, New Delhi for research grant
YSS/2015/000698. MA and AKS thank to MANF, New Delhi and
MNIT Jaipur for fellowships. Authors acknowledge MRC, MNIT
Jaipur for providing characterisation facilities and IIT Indore for
single-crystal X-ray analysis.
DOI: 10.1039/C9OB01674K
1
was subjected to analyse the yield through H NMR. The solid
residue was hydrolysed with KOH (1.68 g dissolved in 10 mL of
EtOH and 5 mL of H2O) at 90 °C for 2 days. The resulting
solution was neutralised with aqueous 20% (v/v) HCl,
extracted with dichloromethane followed by ethyl acetate. The
solvent of the combined extract was evaporated off, and the
resulting residue was analysed with 1H NMR.
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
matter under discussion, limited experimental and spectral data,
and crystallographic data.
Procedure for Suzuki-Miyaura coupling
A reaction tube charged with aryl/heteroaryl halide (1.0
mmol), boronic acid (1.5 mmol), K2CO3 (0.276 g, 2.0 mmol),
water (2.0 mL), and catalyst (0.01 mol%) was placed on an oil
bath maintaining the temperature at 55 °C. The reaction
mixture was stirred and monitored by TLC until the maximum
conversion of the substrate to the cross-coupled product was
reached out. After the optimal conversion, the reaction was
stopped, and the reaction mixture was extracted with ethyl
acetate (2 × 15 mL). The extract was washed with water (2 × 15
mL) and dried over anhydrous Na2SO4. The solvent was
evaporated off on a rotary evaporator, and the crude product
was further purified by column chromatography on silica gel
using ethyl acetate-hexane as eluent. All coupling products
were authenticated by 1H and 13C NMR.
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work as stock of active homogeneous palladium species
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present catalyst was also explored for Suzuki-Miyaura coupling
reaction. It efficiently works in water for the Suzuki-Miyaura
coupling of aryl and heteroaryl bromides and produces yields
up to 94% of coupling product at 55 °C in 2h.
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Conflicts of interest
There are no conflicts of interest to declare.
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Acknowledgements
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